why not just use str_replace i.e.
<?php
$emoji = array(":=)", ":=(", ":sick:");
$emojiImg = array("<img src='smile.png' />", "<img src='sad.png' />", "<img src='sick.png' />");
$string = ":=) hello im :sick: :=(";
$output = str_replace($emoji, $emojiImg, $string);
?>